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Two tables ... an illustration of the different joins
SQL Primer as Used in MySQL example from a Well House Consultants training course
More on SQL Primer as Used in MySQL [link]
Source code: linked.sql Module: S152
# Two tables ... an illustration of the different joins
# In this example (from a MySQL course example I wrote for our delegates) # we create two tables - one of train journeys, and the other of places # to eat at the station prior to departure. # # Since there are a number of possible journeys from each starting point, # I used two tables - one for the journeys, and another to give the best # place at each starting point before the journey. That way, I only have # one set of data to maintain about best eating places, and no duplication # of data. I then use a MySQL JOIN clause in my SELECT to link up the tables. # # A regular JOIN will ONLY give me results where there is somewhere to eat # at the departure station. If I don't have anywhere available to eat, or # if there's somewhere to eat at a station from which I have no journeys, there # will be no record in the output. # # If, on the other hand, I use a LEFT JOIN, my result set will include all of # the joined records for a regular join and IN ADDITION I'll get an additional # record for each row in the left hand table that has no match in the right # hand table, with the joined record being filled in with NULLs. Thus my result # set will have at least one record in it for each and every record in the # incoming left hand table. # # A RIGHT JOIN, similarly, includes at least one record on the output for # each record in the incoming right hand set ... # # So - # * left table: places to eat # * right table: journeys # means # * regular join: Journeys where you can eat before travelling # * left join: all places to eat PLUS all journeys you can make after eating # * right join: all journeys, and places to eat beforehand where available drop table if exists journey; create table journey ( jid int primary key not null auto_increment, start_at text, end_at text, miles int); drop table if exists place; create table place ( pid int primary key not null auto_increment, called text, gridref text, about text); insert into journey values (1,"Chippenham","London",95); insert into journey values (2,"Chippenham","Slough",80); insert into journey values (3,"Llandod","Hereford",35); insert into journey values (4,"Hereford","Newport",40); insert into journey (start_at, end_at, miles) values ("Melksham","Acton",95); insert into journey (start_at, end_at, miles) values ("Melksham","Chippenham",5); delete from journey where jid = 4; replace into journey values (3,"Llandrindod Wells","Hereford",35); update journey set miles = 96 where jid = 1; insert into place (called,gridref,about) values ("Chippenham","SU635443","Nice Chinese"), ("London","TQ543223","Yo Sushi"), ("Westbury","ST998665","Tail of Spice"), ("Melksham","SU654222","Burger Van"); # Two alternative ways of linking tables ... first shows principle BUT # second is the 'proper' and more efficient way: select * from place,journey where called = start_at and miles > 50; select * from place join journey on called = start_at where miles > 50; # +-----+------------+----------+--------------+-----+------------+--------+-------+ # | pid | called | gridref | about | jid | start_at | end_at | miles | # +-----+------------+----------+--------------+-----+------------+--------+-------+ # | 1 | Chippenham | SU635443 | Nice Chinese | 1 | Chippenham | London | 96 | # | 1 | Chippenham | SU635443 | Nice Chinese | 2 | Chippenham | Slough | 80 | # | 4 | Melksham | SU654222 | Burger Van | 5 | Melksham | Acton | 95 | # +-----+------------+----------+--------------+-----+------------+--------+-------+ # 3 rows in set (0.00 sec) # # +-----+------------+----------+--------------+-----+------------+--------+-------+ # | pid | called | gridref | about | jid | start_at | end_at | miles | # +-----+------------+----------+--------------+-----+------------+--------+-------+ # | 1 | Chippenham | SU635443 | Nice Chinese | 1 | Chippenham | London | 96 | # | 1 | Chippenham | SU635443 | Nice Chinese | 2 | Chippenham | Slough | 80 | # | 4 | Melksham | SU654222 | Burger Van | 5 | Melksham | Acton | 95 | # +-----+------------+----------+--------------+-----+------------+--------+-------+ # 3 rows in set (0.00 sec) # DIFFERENCE BETWEEN MATCH TYPES ... # All records that match select * from place join journey on called = start_at; # or use further / full qualification if there may be conflict select * from place join journey on place.called = graham.journey.start_at; # +-----+------------+----------+--------------+-----+------------+------------+-------+ # | pid | called | gridref | about | jid | start_at | end_at | miles | # +-----+------------+----------+--------------+-----+------------+------------+-------+ # | 1 | Chippenham | SU635443 | Nice Chinese | 1 | Chippenham | London | 96 | # | 1 | Chippenham | SU635443 | Nice Chinese | 2 | Chippenham | Slough | 80 | # | 4 | Melksham | SU654222 | Burger Van | 5 | Melksham | Acton | 95 | # | 4 | Melksham | SU654222 | Burger Van | 6 | Melksham | Chippenham | 5 | # +-----+------------+----------+--------------+-----+------------+------------+-------+ # 4 rows in set (0.00 sec) # # +-----+------------+----------+--------------+-----+------------+------------+-------+ # | pid | called | gridref | about | jid | start_at | end_at | miles | # +-----+------------+----------+--------------+-----+------------+------------+-------+ # | 1 | Chippenham | SU635443 | Nice Chinese | 1 | Chippenham | London | 96 | # | 1 | Chippenham | SU635443 | Nice Chinese | 2 | Chippenham | Slough | 80 | # | 4 | Melksham | SU654222 | Burger Van | 5 | Melksham | Acton | 95 | # | 4 | Melksham | SU654222 | Burger Van | 6 | Melksham | Chippenham | 5 | # +-----+------------+----------+--------------+-----+------------+------------+-------+ # 4 rows in set (0.00 sec) # All records that match PLUS orphans in left / right table select * from place left join journey on called = start_at; select * from place right join journey on called = start_at; # +-----+------------+----------+---------------+------+------------+------------+-------+ # | pid | called | gridref | about | jid | start_at | end_at | miles | # +-----+------------+----------+---------------+------+------------+------------+-------+ # | 1 | Chippenham | SU635443 | Nice Chinese | 1 | Chippenham | London | 96 | # | 1 | Chippenham | SU635443 | Nice Chinese | 2 | Chippenham | Slough | 80 | # | 2 | London | TQ543223 | Yo Sushi | NULL | NULL | NULL | NULL | # | 3 | Westbury | ST998665 | Tail of Spice | NULL | NULL | NULL | NULL | # | 4 | Melksham | SU654222 | Burger Van | 5 | Melksham | Acton | 95 | # | 4 | Melksham | SU654222 | Burger Van | 6 | Melksham | Chippenham | 5 | # +-----+------------+----------+---------------+------+------------+------------+-------+ # 6 rows in set (0.00 sec) # # +------+------------+----------+--------------+-----+-------------------+------------+-------+ # | pid | called | gridref | about | jid | start_at | end_at | miles | # +------+------------+----------+--------------+-----+-------------------+------------+-------+ # | 1 | Chippenham | SU635443 | Nice Chinese | 1 | Chippenham | London | 96 | # | 1 | Chippenham | SU635443 | Nice Chinese | 2 | Chippenham | Slough | 80 | # | NULL | NULL | NULL | NULL | 3 | Llandrindod Wells | Hereford | 35 | # | 4 | Melksham | SU654222 | Burger Van | 5 | Melksham | Acton | 95 | # | 4 | Melksham | SU654222 | Burger Van | 6 | Melksham | Chippenham | 5 | # +------+------------+----------+--------------+-----+-------------------+------------+-------+ # 5 rows in set (0.00 sec) # JUST orphans in left / right table # next line is WRONG ... select * from place left join journey on called = start_at where jid = NULL; # # Empty set (0.00 sec) # # instead, you want ... select * from place left join journey on called = start_at where jid is NULL; # +-----+----------+----------+---------------+------+----------+--------+-------+ # | pid | called | gridref | about | jid | start_at | end_at | miles | # +-----+----------+----------+---------------+------+----------+--------+-------+ # | 2 | London | TQ543223 | Yo Sushi | NULL | NULL | NULL | NULL | # | 3 | Westbury | ST998665 | Tail of Spice | NULL | NULL | NULL | NULL | # +-----+----------+----------+---------------+------+----------+--------+-------+ # 2 rows in set (0.00 sec) # and select * from place right join journey on called = start_at where pid is NULL; # +------+--------+---------+-------+-----+-------------------+----------+-------+ # | pid | called | gridref | about | jid | start_at | end_at | miles | # +------+--------+---------+-------+-----+-------------------+----------+-------+ # | NULL | NULL | NULL | NULL | 3 | Llandrindod Wells | Hereford | 35 | # +------+--------+---------+-------+-----+-------------------+----------+-------+ # 1 row in set (0.00 sec) # http://dev.mysql.com/doc/refman/5.5/en/reserved-words.html # Example that's sorted ... select * from place right join journey on called = start_at order by called desc, miles; # +------+------------+----------+--------------+-----+-------------------+------------+-------+ # | pid | called | gridref | about | jid | start_at | end_at | miles | # +------+------------+----------+--------------+-----+-------------------+------------+-------+ # | 4 | Melksham | SU654222 | Burger Van | 6 | Melksham | Chippenham | 5 | # | 4 | Melksham | SU654222 | Burger Van | 5 | Melksham | Acton | 95 | # | 1 | Chippenham | SU635443 | Nice Chinese | 2 | Chippenham | Slough | 80 | # | 1 | Chippenham | SU635443 | Nice Chinese | 1 | Chippenham | London | 96 | # | NULL | NULL | NULL | NULL | 3 | Llandrindod Wells | Hereford | 35 | # +------+------------+----------+--------------+-----+-------------------+------------+-------+ # 5 rows in set (0.00 sec) Learn about this subject
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